Conventional onboard travel control systems have been in use for a number of years, at least in high-end vehicles. The travel control systems include a navigation system to enable the vehicle's present position to be determined. For this purpose, satellite navigation is generally combined with an evaluation by vehicle sensors. Travel control systems further include at least one digital road map that is part of the travel control system, for example in the form of a CD-ROM. The onboard travel control systems operate autonomously, but are not able to take into account up-to-date traffic information.
A method for transmitting traffic information in a standardized protocol is known from Preliminary European Standard ENV12313-1. This method uses location codes that can be used to identify all of a country's traffic-related nodes that are listed in a location table. In addition to location coding, the protocol also codes the direction and extent of the route, thereby making it possible to define the territory described by the traffic disturbance. The traffic information is transmitted in the radio data system (RDS) along with a radio broadcast and can be stored, decoded, and output in a suitable form as a traffic message channel (TMC) in the receiver. To enable the travel control system to take the traffic information into account, the digital maps used in the travel control system are provided with the information in the location table, thereby determining whether an up-to-date item of traffic information affects the pre-calculated route in the case in hand. The onboard travel control system is able to automatically use the traffic information only to a limited extent, due to the considerable cost that this involves. The system does not take into account traffic forecasts and can take into account only events limited to the TMC locations for route planning purposes.
Attempts have therefore been made to use a central computer for performing a route calculation that takes into account the up-to-date traffic information and to supply the route calculation as a service, for examples in a mobile radio system. For this purpose, the onboard travel control system must be provided with a remote data transmission system. To interpret the centrally calculated, optimized route in the vehicle itself, the central office must use an easy-to-understand digital map. According to a method of this type used by Mercedes Benz in Tokyo, the route is transmitted by arranging the route components to be traveled in sequential order. The identifiers for the route components are then transmitted in a code that can be used only in Tokyo. This has a particular disadvantage in that, with complex routes, the transmission capacity required increases in more or less linear proportion to the length of the route. The transmission capacity requires the use of expensive point-to-point data connections, with the transmission of a route for a trip within the city of Tokyo requiring transmission times of at least 45 seconds. Data transmissions of such a duration cannot be reliably ensured while driving a vehicle.